In the coming years an increasing quantity of vacuum interrupters (VI) is expected to reach the end of its service life. Leading manufacturers can provide a service life of up to 30 years due to high quality standards and sophisticated production processes. However, their operating performance observed so far indicates the possibility to extend this service life, in case of non-intrusive diagnosis methods being available for the assessment of the remaining service life and the simultaneous estimation of the technical risks. Amongst other factors especially the internal pressure is decisive for the condition of the VI. However, currently available test devices only provide qualitative information, which is insufficient to provide a basis for the assessment of the condition of vacuum switchgear and for a decision concerning potential extension of the service life. This contribution addresses a mobile test device based on the magnetron method for the quantitative assessment of the internal pressure in VIs. It identifies the magnetic flux density and its homogeneity as critical influencing factors in the design of a mobile magnetron-based test device. Based on the results, a simplified, exemplary prototype for on-site application is designed. The application of this device on industrially manufactured VIs verifies its functionality and generally demonstrates the proof of concept.

Despite intensive research work during the last decades, the interruption process in high-voltage circuit breakers is still not fully understood. Especially with regard to the adaption of today’s switching technologies for SF6-substitutes as e.g. CO2 (carbon dioxide) an improved understanding of the physical processes during current interruption is required. Here optical emission spectroscopy (OES) offers time-resolved access to physical properties, e.g. temperature and pressure, in the nozzle system. Thus OES measurements are performed by means of a circuit breaker model in this contribution. The obtained arc temperature profiles and temperature characteristics are in good agreement with the results of CFD simulations (computational fluid dynamics).

The magnetron method is well established for the internal pressure measurement of vacuum interrupters. Its application during the manufacturing process is essential for the determination of the vacuum quality. The evaluation of the magnetron measurement requires precise knowledge of the residual gas composition and the influence of the metallic surfaces in the vacuum interrupter due to gas binding effects. Leading manufacturers having gained wide experience applying the magnetron method do consider these influences during the production process. However, it is also possible to measure the internal pressure in a vacuum interrupter by means of the magnetron method after a considerable long time in service, e.g. close to the end of the prospective service life. As the residual gas composition in these states is not precisely known, uncertainties in the pressure measurement can arise. Experiments with common residual gases and their typical compositions, which can form inside a vacuum interrupter, show that the uncertainty is less than one decade. In addition experiments performed on vacuum interrupters identify the temporary gas binding due to the Getter-Ion Effect as a factor that considerably limits the repeatability of the magnetron measurement.

The maintenance of high voltage gas circuit breakers requires high personal as well as monetary efforts for the asset operator. Furthermore, a faulty reassembly of a circuit breaker during maintenance can lead to a circuit breaker failure during operation. One possibility to reduce those efforts as well as the failure risk is the application of non-invasive diagnostic techniques. This research work examines such a non-invasive technique for assessing the wear of the insulating nozzle inside the switching chamber of a circuit breaker. The approach applied is based on the measuring of the transient pressure signal at the gas connector of the circuit breaker during a switching operation without electrical load. The pressure signal is investigated regarding characteristically features which yield information for the determination of the condition of the switching chamber. This contribution addresses two different approaches for the evaluation of the measured pressure signals: firstly a comparative analysis of the signals and secondly an evaluation based on machine learning algorithms. Both of them use a dataset containing measurements with different variations of the nozzle shape representing the wear of the nozzle. In the first approach, the characteristics of measurements with different nozzle diameters are compared and assessed with regard to deviations of the characteristics. In the second approach, machine learning algorithms are used to automatically scan for dependencies in the given dataset obtained from the measurement data. Applied on the results of exemplary tests, both approaches provide a clear indication with regard to a prediction of the nozzle wear and show a proof of concept for the new non-invasive diagnostic technique. Taking into account a large database from field tests planned in the future, especially the machine learning algorithms can show a valuable benefit for the application of this test measure during circuit breaker maintenance.

A switching arc in a model circuit breaker is studied by means of CFD simulations and optical emission spectroscopy. The arc is initiated between tungsten-copper electrodes in a carbon dioxide atmosphere and is led through PTFE (polytetrafluorethylene) nozzles. Radiation emitted by the arc plasma is absorbed by the surface of the nozzles leading to ablation of the wall material. The CFD simulations are based on a model of the arcing zone including a consistent treatment of the radiation transport and wall ablation. Carbon ion line radiation is analysed in the experiment using an optical path in the heating channel between the nozzles. Temperature profiles obtained from spectroscopy and simulations are compared. The pressure value in the arc is estimated based on the line width-intensity dependence. The obtained values correspond to the measured pressure outside the arc. Coincidence in temperature for the peak current and discrepancy on the falling edge are found and discussed.

In standard high voltage gas circuit breakers -important safety elements in today's power grids-, a small surface layer of polytetrafluoroethylene (PTFE) vaporizes at the high temperatures occurring in the arcing zone during the high current phase of the interruption process. This ablation continues even some hundred microseconds after the arc has been quenched and it actually changes the total gas composition as well as the temperature profile in the arcing zone. Therefore it delays the fast cooling of the arcing zone, which is necessary to prevent dielectric failures. The experimental determination of the dielectric recovery of hot air in insulating nozzles was investigated previously with focus on the substitution of SF6 (sulfur hexafluoride) in circuit breakers for future power grids. The present investigation gives a physical model for the calculation of the previously measured recovery characteristics. CFD-simulations (computational fluid dynamics) are performed for the determination of the relevant physical properties of the decaying quenching gas (density, temperature etc.). These properties serve as input parameters for the developed model. Based on the gas properties resulting from the CFD-simulations it is possible to calculate the effective ionization coefficients and thus the breakdown voltage applying the streamer criterion. Afterwards the calculated breakdown voltages are compared to the measured recovery characteristics. The comparison shows a good agreement between the measurements and the calculation.

The most common filling gas of self-blast circuit breakers is SF6. If one wants to replace it by a substitute it has to be investigated whether the physical models which have been derived and proofed for SF6-filled circuit breakers are valid for non-SF6 breakers as well. Here the influence of the pressure of the blowing gas on the thermal interruption capability is of interest. First a non-SF6 self-blast circuit breaker model is designed by means of CFD-Simulations. Second this circuit breaker model is used to determine the thermal interruption capability at different blow gas pressures. From these results the validity of the relationship between the blow gas pressure and the thermal interruption capability can be derived.

In self-blast circuit breakers the material ablating from cylindrically shaped nozzles causes a pressure build-up inside a heating volume. Shortly before current zero the decreasing pressure inside the insulating nozzles leads to a backflow of gas from the heating volume which cools the arc. Circuit breaker prototypes are used to focus on the main influencing parameters as for example the pressure build-up and the amount of ablated nozzle material. In this paper the influence of alternative nozzle geometries on the switching behaviour of a circuit breaker prototype is investigated. The focus is on nozzle geometries with an additional ablation element. The results are compared to the results of previous investigations with conventionally shaped nozzles as well as to the results of CFD-simulations (computational fluid dynamics).

Optical emission spectroscopy (OES) was applied for diagnostics of switching arcs in a model circuit breaker test device with polytetrafluoroethylene (PTFE) nozzles. Carbon dioxide (CO2) at ambient pressure (1 bar) was used as insulating and quenching gas. An electrical arc was ignited between the contacts of this breaker. A current with an amplitude of 5.5 kA and a frequency of 50 Hz were chosen. The spectra observed were dominated by the radiation of ablated nozzle material. The analysis of spectroscopic lines of carbon ion allowed the determination of temperature profiles and an estimation of the pressure in several phases of the current half cycle. Recordings of a high-speed camera gave an additional qualitative picture of the arc development. Furthermore, the correlation in the appearance of the arc with the peculiarities of the measured arcing voltage was identified.